Seed and seedling traits suggest ontogenetic coordination in the functional recruitment niche for dryland restoration species

Plant recruitment is shaped by functioning across seed and seedling stages. Because seed morphology and germination directly influence seedling exposure to resources and environment, these two stages may be linked through trait synergies and trade-offs that coordinate functioning through early ontogeny. However, the wide range of traits impacting environmental response at each ontogenetic stage are rarely explored in tandem to understand the potential dimensionality of the functional recruitment niche. We explored covariation among 13 seed and seedling traits linked to stress tolerance, rate of germination or growth, light response, temperature response and other functions for 49 species found in semi-arid rangelands. Using phylogenetically informed ordination and cluster analysis, we asked how trait covariation across multiple ontogenetic stages and functions shapes the dimensionality of the functional recruitment niche. The first two trait dimensions identified at separate seed and seedling stages aligned, providing some basis for ontogenetic coordination during recruitment. Morphological traits reflecting size-related stress tolerance (i.e. seed and seedling mass) formed the strongest foundation for coordination across stages, sharing ties with traits reflecting seedling light response (specific leaf area), growth rate (root elongation) and seed temperature response (e.g. germination minimum temperature). We also observed an unexpected trade-off in how seeds and seedlings may avoid risk (through dormancy) or tolerate risk (through root investment), respectively. In contrast, seed light response, seed germination rate and seedling minimum temperature thresholds were not tightly linked to analogous functions at other stages. Their independence could expand the dimensionality of the recruitment niche depending on the functional significance of these traits in the field. Synthesis. Seed and seedling stages are characterized by multiple, independent dimensions of functioning, but ontogenetic coordination may moderate increasing dimensionality of the functional recruitment niche as a wider breadth of traits are explored together. At the same time, physiological traits linked to environmental response appear less connected to other traits and could complexify spatiotemporal recruitment dynamics. Both the independent and coordinated aspects of functioning observed here deserve exploration across a broader range of species, traits and environments to understand the full dimensionality of the functional recruitment niche.